1. Technical Field
The present invention relates to a wide angle lens system for photographic applications and in particular for digital single lens reflex cameras.
2. Related Background Art
Wide angle lenses capable of capturing 175° or more angle of view are useful for creating panoramic images. Digital Single Lens Reflex (DSLR) cameras are gaining popularity both for the amateur and professional photographer as the electronics have provided improved image quality. Cameras using 10 megapixel sensors and improved electronics have resulted in image quality that can rival and even exceed that of conventional 35 mm film cameras. The physical dimensions of the DSLR camera sensors are however not always identical to the dimensions used for film based cameras. Digital cameras require new designs for lenses. The lenses must accommodate image size requirements for the digital sensors, and back focal length constraints based upon the dimensions of the cameras and its internal mechanics and still provide exceptional image quality and optical performance as defined in terms of low aberrations. These combined requirements have provided new challenges in wide angle lens design. Additionally the advances in electronics have resulted in a reduction in costs for advanced photographic systems. The reduced costs for cameras have placed increased pressure on lens designers and manufacturers to also reduce cost. A primary means to reduce cost is to design lenses that provide the same performance with fewer lens elements.
Some DSLR cameras use CMOS or CCD imagers that are smaller in effective area than a 35 mm-film frame which has an active area of approximately 36 mm×24 mm. For example, the Canon® 350D camera uses a CMOS imager with approximately 23×15 mm image area (known as “APS-C” format). The Nikon® D40x uses a CCD imager with a similar effective area (known as “DX” format). In order to form a complete circular image with such DSLR cameras, the lens image circle must be less than the vertical height of the sensor. This, in turns, requires that the focal length or effective focal length (EFL or f) of the wide angle lens is about 6 mm or shorter.
The structure of the camera dictates that the back focal distance (BFL) of the wide angle lens must be sufficiently long in order to avoid colliding with the mirror inside the camera body. The short EFL and long BFL requirement makes the wide angle lens design difficult.
Existing lens designs such as those disclosed in U.S. Pat. Nos. 6,844,991 and 3,737,214 and 3,741,630 and Japanese Patent JP60-153018 do not fulfill this requirement of long BFL and short EFL for DSLR cameras having APS-C or DX format sensors. The inventor is aware of only one other commercially available wide angle lens made by Coastal Optical Systems (http://www.coastalopt.com/stan—01c.asp) that fulfills the dimensional requirements for the DX format cameras. However, the Coastal lens design is large and complex (it has 10 internal lens elements) and thus expensive to produce.
There is a therefore a need for a wide angle lens that has a field of view of at least 175° with excellent imaging characteristics and low cost to manufacture. There is a need to provide a wide angle lens design with at least a 175° field of view that provides a full circular image on an APS-C or DX format imagers. There is a need for lenses that simultaneously provide at least a 175° field of view, a full circular image on an APS-C or DX format imagers, have excellent image characteristics and a low cost to manufacture.